1. Field of the Invention
This invention relates to optical objectives of variable focal length having at least two axially movable lens units for variation of the focal length
2. Description of the Related Art
In general, zoom lenses need, besides good correction of aberrations in the standard position, to have another requirement of maintaining the aberration correction as stable throughout the entire zooming range as possible Hence, there is need to correct all the lens units individually for spherical aberration, coma and astigmatism. It has, therefore, been the common practice to use several lens elements in constructing any one of the lens units.
Recently, there has been increasing demand for a much-desired reduction of the bulk and size of the zoom lens along with a greater zoom ratio. To decrease the longitudinal length of the zoom lens, paraxially speaking, either the power of each lens unit may be strengthened, or the interval between the principal points in each pair of adjacent two lens units be shortened. To extend the zooming range, if it is only at the paraxial zone that good stability of image aberrations may be established, either the power or the axial movement of each zoom unit can be increased.
To fulfill both requirements of achieving a great advance in the compactness of the zoom lens and a great increase of the zoom ratio, as far as the paraxial zone is concerned, to be sure, the choice of the method of stregthening the power of each lens unit is recommendable. But, for an actual useful zoom lens with good image quality over the entire area of the image format under the condition that the power of each lens unit is strengthened, the necessary number of lens elements in each unit must be greatly increased to permit limitation of produced aberrations to a minimum. If so, the overall axial thickness of the lens unit will increase. This calls for an increase in the principal point interval, leading to an impossibility of shortening the longitudinal length of the entire system. Such increase in the overall axial thicknesses of the lens units also causes decrease of the axial length of the space in which the variator lens unit moves. A valuable increase in the zoom ratio will, therefore, become impossible to achieve. Further, as the overall axial thickness of either the first or front, or the second lens unit increases, the passage of an oblique beam as large as the axial beam is permitted, the diameter of the front lens unit must be increased. Thus, the complete objective will become bulkier. Since there is such a bad endless chain, so long as the conventional spherical lens system is not drastically altered, the problem of achieving a much-desired advance in the compactness with a great increase in the zoom ratio in such a manner that all aberrations can be well corrected, cannot be solved.
Among others, the Petzval sum becomes very difficult to correct when it is by strengthening the power of each lens unit that the total optical length of the entire lens system, or the distance from the front vertex to the image plane, is shortened.
In this connection, taking an example of the wellknown 4-unit zoom lens, which the positive first, negative second and positive or negative third lens units, counting from front, constitute a zoom section, and the positive fourth lens unit constitutes a relay section, it should be pointed out that there are two methods of shortening such a zoom lens in the longitudinal direction, namely (1) by strengthening the power of each of the lens units in the zoom section, or (2) by decreasing the telephoto ratio of the relay section. According to (1) method, the second lens unit, as the variator which is usually strongest in the negative sense of power, takes a large value of the Petzval sum with the result that curvature of field is extremely overcorrected. In (2) also, the decrease in the telephoto ratio of the relay lens tends to change the sign of the value of Petzval sum to the negative one. The use of the latter method, therefore, also results in over-correction of field curvature.
In aiming to correct the Petzval sum, if the refractive index of the positive lens is lowered, or a positive lens of strong power is combined with a negative lens, very large spherical aberration or higher order aberration is, in turn, produced which cannot be corrected by the suitable design of the other lens units. It is to be understood that the reducing of the size of the zoom lens is incompatible with good correction of Petzval sum as far as the spherical lens system is concerned.
Such a situation is not confined to the aforesaid kind of zoom-unit configuration, but is valid to other kinds of configuration such as those in which the positive first lens unit axially moves forward when zooming from the wide angle to the telephoto end, or the fourth lens unit also axially moves during zooming.
An attempt has been made to get around this difficulty by employing a compound-eyes optical system having a function of forming an erected image of unity magnification in place of the relay lens unit. In this method, the relay lens unit is constructed with a lens of simple form and a network of a great number of self-convergent spherilenslets or barlenslets. Though the relay lens unit itself is of simple structure, because the means for relaying the image formed at a constant position by the zoom section to the last image plane is in the form of compound eyes, there are many restrictions.
In FIG. 41 there is shown a prior known zoom lens of the kind in which the two air separations between the successive two of positive first, negative second and negative third lens units A, B and C are varied to vary the focal length while compensating for the image shift. As is obvious from the drawing, usually the unit A is necessarily of the largest diameter among the four lens units A to D and, because of its having two or more members, is very heavy. In this connection it should be explained that the positive front unit A is designed with the prerequisites that the on-axis F-number has a prescribed value in the telephoto end, and the diameter is so large as to admit of the light beams that proceed to the corners of the image frame even when in the wide angle end, and that to allow for the zoom ratio to be taken at a desired value, an object point for the negative second unit B is formed in an appropriate position, and its focal length of positive sign is so short as to be suited to a prescribed focusing range. Meanwhile, to assist in stabilizing aberrations during zooming, the unit A is corrected very often in itself for spherical aberration including chromatic aberrations with a greater emphasis than on the other aberrations. Under such a circumstance, therefore, so long as the designer relied on the employment of a single lens for the unit A, it has been impossible to realize an achievement of the much-desired reduction of the bulk and size of the unit A, since among the homogeneous optical materials there exists no one that allows for sufficient correction of chromatic aberrations. Another method of constructing the unit A from two lenses of which the positive lens is made of a material of relatively low dispersion and another lens which is negative and made of relatively high dispersive material, has also sometimes been employed. But the addition of such a negative lens into the unit of positive overall power requires that the focal length of the positive lens be short enough to cancel the power of the negative lens, or its surface curvature becomes stronger. To hold the prescribed value of the diameter, therefore, the minimum acceptable axial thickness of the positive lens is caused to rapidly increase. Correcting spherical aberration also is limited to the extent of the possible amount of correction by the two spherical lenses. In many cases, therefore, it has been unavoidable to introduce an aspherical lens.
That the increase of the axial thickness of the first or second lens unit of the zoom lens gives a serious problem to the advance in compactness has already been mentioned. Particularly to the second unit, when its power is increased in the negative sense, because of the necessity of increasing the necessary number of constituent lens elements, its overall axial thickness tends to increase rapidly.
In another zoom lens comprising at least three lens units, or, from front to rear, a positive first lens unit, a negative second lens unit, and a third lens unit of strong power, the zooming arrangement being such that as zooming from the wide angle to the telephoto end, the axial separation between the first and second units increases and the axial separation between the second and third units first descreases and then increases, the method of minimizing the total optical length is either to strengthen the power of the third unit, or to shorten the interval between the principal points of the second and third units. However, when the power of the second unit is strengthened, the necessary number of lens elements in the second unit to correct aberrations increases with increase in the overall axial thickness thereof. Therefore, the principal point interval must be increased and the total optical length of the entire system cannot be appreciably shortened. Also when the power of the positive third unit is strengthened, all those aberrations which arise within the positive third lens unit, in most cases, particularly spherical aberration and astigmatism, are caused to increase. To correct these aberrations to desired values, it is unavoidable to increase the number of constituent lens elements of the positive third unit.
Meanwhile, as to reduce the size of the relay section of the zoom lens, mention may be made of the method of strengthening its telephoto form. To do this, the positive power must be concentrated in the front part of the relay section, while the negative power at the rear part. With the use of this method, however, because the front and rear parts are necessarily spaced apart to allow for the decreasing of the telephoto ratio of the relay section, and because the number of the ones of the lens elements whose powers are strong enough to correct curvature of field, spherical aberration and distortion becomes large, a very long physical length of the relay lens unit has resulted. In fact, despite the minimum possible back-focal distance, there has been a limitation on the extent of reduction of the distance from the front vertex of the relay section to the image plane.
Further, since the zoom lens has its relay unit made of a largest number of constituent lens elements among all the units and also since each element has a strong power, it takes a long time and great effort to manufacture such lens elements and mountings therefor, and, because a severe tolerance must be assigned to the axial alignment of the lens elements, another long time is required for the adjusting operations on the assembly line. Also since the total sum of the constituent lens elements over the zoom lens is very large, there are produced many adverse effects to improvements of the image quality, such as those of surface reflections, ghost light and flare spots, which are peculiar to the zoom lenses.